1. Field of the Invention
This invention relates to a light scanning device using a polygon mirror.
2. Description of the Prior Art
Scanning a light beam by a light scanning device using a polygon mirror has been widely practised but it is very difficult to enhance the accuracy of the repetitive scanning position of the device without the use of correcting means. More specifically, when a light beam is deflected by a light scanning device using a polygon mirror, it is preferable to ensure the deflected light beam to scan always on the same locus on a member to be scanned. However, the polygon mirror and the rotary shaft for mounting it thereon have been machined separately from each other, whereafter they have been firmly coupled together for rotation as a unit. Thus, the machining errors of the polygon mirror and the rotary shaft have been brought out in a combined form which has increased the deviation of the repetitive scanning position of the light scanning device. The light scanning device using a polygon mirror according to the prior art will further be discussed by reference to FIG. 1 of the accompanying drawings.
A rotary shaft 1 rotatably supported by means of bearings 2 such as ball bearings or the like has integrally formed therewith a flanged portion 1a having a reference surface 1b for supporting thereon a polygon mirror 3. The polygon mirror 3 supported on the flanged portion 1a, as already noted, comprises a glass (or metal) block having the shape of, for example, a regular octagon, and formed with a through-aperture in the center thereof for mating with the rotary shaft 1. Each of the eight surfaces forming the octagon is formed as a mirrored surface parallel to the rotary shaft 1.
To enable the polygon mirror 3 to be rotated with the rotary shaft 1, the polygon mirror 3 is placed on the reference surface 1b of the flanged portion 1a with the underside of the polygon mirror as a reference surface 3a, and a washer 8 is overlaid on top of the polygon mirror 3, and a nut 9 threadably engageable with the externally threaded portion of the rotary shaft 1 is tightened to fix the polygon mirror to the rotary shaft. The polygon mirror 3 fixed to the rotary shaft 1 is dust-proofed by a casing 4, a portion of which is formed as a window 5 through which the light beam B may enter the reflecting surfaces of the polygon mirror 3 and be reflected outwardly thereby.
A pulley 6 is secured to one end of the rotary shaft 1 and may be rotatively driven from a motor, not shown, through a belt 7 connected to the motor, so that the incident light beam may be deflected to exit and scan.
In such light scanning device of the prior art, the machining precision of perpendicularity of the reference surface 1b of the flange to the center axis 0--0' of the rotary shaft 1 would suffer from an error of the order of .+-.3 seconds (of angle) if the existing machining method is resorted to, even though the machining was effected with considerable care carefully. Also, the machining precision of perpendicularity of each mirrored surface to the reference surface 3a of the polygon mirror 3 would likewise suffer from an error of the order of .+-.2 seconds. In addition to such machining errors of the two members, the light scanning device made by combining the two members involves an error attributable to the bearing precision of the rotary shaft and thus, the precision of the device as a whole is usually lower than the precision of each individual member forming the device.
Therefore, in the existing light deflecting device, an error of the order of .+-.5 seconds with respect to an ideal reflected light beam B0 has been the limit for the error between light beam B1 and B2 reflected from the respective reflecting surfaces of the polygon mirror.
In addition, such error has been greatly varied by the precision of assembly.